IN DESCRIPTIVE ASTRONOMY. 117 



is 1 hr. 50 min. At midwinter it is longer by about seven- 

 teen minutes ; but the augmentation is frequently not percepti- 

 ble, owing to the greater prevalence of clouds and haze at 

 that season of the year, which intercept the light, and hinder 

 it from reaching the earth. The duration is least at the equa- 

 tor (1 hr. 12 min.), and increases as we approach the poles ; 

 for at the former there are two twilights every twenty-four 

 hours, but at the latter only two in a year, each lasting about 

 fifty days. At the north pole the sun is below the horizon for 

 six months, but from January 29 to the vernal equinox, and 

 from the autumnal equinox to November 12, the sun is less 

 than 18 below the horizon ; so that there is twilight during 

 the whole of these intervals, and thus the length of the actual 

 night is reduced to 21 months. The length of the day in these 

 regions is about six months, during the whole of which time 

 the sun is constantly above the horizon. The general rule is, 

 that to the inhabitants of an oblique sphere the twilight is longer in 

 proportion as the place is nearer the elevated pole, and the sun is 

 farther from the equator on the side of the elevated pole. Cham- 

 bers' Astronomy. 



When the sun rises or sets most obliquely to the horizon, 

 then the least time is required to pass through the necessary 

 18, and, of course, the length of twilight is the least. When 

 the sun rises or sets least obliquely, the most time is required 

 to pass through 18, and the length of twilight is greatest. If 

 the sun's path is perpendicular to the horizon, the sun will 

 pass over the 18 in 1 hr. 12 min.; for 15= 1 hr.; and hence 

 18 =1^ hr. 



43. What is a moon ? 



A moon is a secondary body, or satellite, revolving about a 

 primary body, or planet. 



44. To a person in the south temperate zone, where 

 would the sun be at noon ? 



On the meridian north of the observer. 



45. Is it correct to say that the moonrevolvcs about the 

 earth 9 ivhen ive know tit at f according to the law of Phys- 



